Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. High-energy ion implanters are used for deep implants into a substrate. Such deep implants are required to create, for example, retrograde wells. Eaton GSD/HE and GSD/VHE ion implanters are examples of such high-energy implanters. These implanters can provide ion beams at energy levels up to 5 MeV (million electron volts). U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, Eaton Corporation, and describing such an high-energy ion implanter, is incorporated by reference herein as if fully set forth.
A block diagram of a typical high-energy ion implanter 10 is shown in FIG. 1. The implanter 10 comprises three sections or subsystems: a terminal 12 including an ion source 14 powered by a high-voltage supply 16 to produce an ion beam 17 of desired current and energy; an end station 18 which contains a rotating disc 20 carrying wafers W to be implanted by the ion beam; and a beamline assembly 22, located between the terminal 12 and the end station 18, which contains a mass analysis magnet 24 and a radio frequency (RF) linear accelerator (linac) 26. The beamline assembly 22 conditions the ion beam output by the terminal 12 and directs the conditioned beam toward the target wafer W. A final energy magnet (not shown in FIG. 1) may be positioned between the linac 26 and the rotating disc.
The mass analysis magnet 24 functions to pass only ions of an appropriate charge-to-mass ratio to the linac. The mass analysis magnet is required because the ion source 14, in addition to generating ions of appropriate charge-to-mass ratio, also generates ions of greater or lesser charge-to-mass ratio than that desired. Ions having inappropriate charge-to-mass ratios are not suitable for implantation into the wafers W.
The ion beam 17 passes through the mass analysis magnet 24 and enters the RF linac 26 which imparts additional energy to the ion beam passing therethrough. The RF linac produces particle accelerating fields which vary periodically with time, the phase of which may be adjusted to accommodate different atomic number particles as well as particles having different speeds. The RF linac 26 comprises a series of resonator modules 30a through 30n, each of which functions to further accelerate ions beyond the energies they achieve from a previous module.
FIG. 2 shows a known type of resonator module 30, comprising a large inductive coil L contained within a resonator cavity housing 31 (i.e., a "tank" circuit). A radio frequency (RF) signal is capacitively coupled to a high-voltage end of the inductor L via capacitor C.sub.C. An accelerating electrode 32 is directly coupled to the high-voltage end of the inductor L. Each accelerating electrode 32 is mounted between two grounded electrodes 34 and 36, and separated by gaps 38 and 40, respectively. C.sub.S represents the stray capacitance of the high-voltage acceleration electrode 32 to ground. R.sub.L represents the losses associated with the resonant circuit comprising L and C.sub.S in a series loop (see FIG. 3).
Values for C.sub.S and L are chosen for the circuit to achieve a state of resonance so that a sinusoidal voltage of large magnitude may be achieved at the location of the accelerating electrode 32. The accelerating electrode 32 and the ground electrodes operate in a known "push-pull" manner to accelerate the ion beam passing therethrough, which has been "bunched" into "packets". During the negative half cycle of the RF sinusoidal electrode voltage, a positively charged ion packet is accelerated (pulled by the accelerating electrode 32) from the first grounded electrode 34 across gap 38. At the transition point in the sinusoidal cycle, wherein the electrode 32 is neutral, the packet drifts through the electrode 32 (also referred to as a "drift tube") and is not accelerated.
During the positive half cycle of the RF sinusoidal electrode voltage, positively charged ion packets are further accelerated (pushed by the accelerating electrode 32) toward the second grounded electrode 36 across gap 40. This push-pull acceleration mechanism is repeated at subsequent resonator modules having accelerating electrodes that also oscillate at a high-voltage radio frequency, thereby further accelerating the ion beam packets by adding energy thereto. The RF phase of successive accelerating electrodes in the modules is independently adjusted to insure that each packet of ions arrives at the appropriate gap at a time in the RF cycle that will achieve maximum acceleration.
FIG. 3 shows the equivalent circuit of the resonator module 30 of FIG. 2. The time dependent input/output variables are voltage v(t) and current i(t). By taking advantage of the duality of time and frequency domain representation (the Fourier transform), time may be eliminated as a variable and replaced with .omega., the radian frequency. In the harmonic steady state of resonance, v(t) and i(t) at frequency f are linearly related by the complex impedance Z(.omega.), such that V=Z(.omega.)I, where v(t)=V sin .omega.t and .omega.=2.pi.f.
In the circuit of FIG. 3, the complex impedance Z of capacitor C.sub.S is proportional to 1/f, with I leading V by 90.degree.; the complex impedance Z of inductor L is proportional to f, with I lagging V by 90.degree.; and the resistive losses R.sub.L are generally independent of frequency, with I and V in-phase with each other. At resonance, maximum voltage is achieved at the accelerating electrode 32 for a given input RF signal, the currents in C.sub.S and L cancel because they are 180.degree. out of phase, and all power in the circuit is dissipated through resistor R.sub.L. To attain a resonant state, .omega.=2.pi.f=(LC).sup.-1/2. For example, in the Eaton GSD series, .omega.=13.56 megahertz (MHz).
To maintain a state of resonance, the product of L.times.C.sub.S must remain constant. The quality factor Q of the resonant circuit also depends upon the ratio of R.sub.L /X, where X=.omega.L, or the ratio of stored energy per cycle over dissipated energy per cycle. Accordingly, drifts in C.sub.S and changes in L during operation may be accommodated by altering only one of these factors, in this case L, to "tune" the resonator circuit. Also, in order to obtain maximum power out of the resonator module 30, the impedance of the resonator circuit must "match" that of the RF input source to minimize reflection of the input signal from the circuit back into the source.
FIG. 4 shows a prior art resonator module and the mechanisms provided for matching and tuning of the resonator circuit. The tuning mechanism comprises a servomotor (not shown) which moves a stem 44 of inductor L in and out of resonator cavity housing 31 in the directions shown by arrow 46. By moving (stretching or compressing) the inductive coil L along axis 47, the inductance value of the inductor can be altered. A collar 48 is provided at the high-current (up to 200 amps), low-voltage end of the inductor, through which the inductor stem slides in and out. However, this tuning mechanism provided in FIG. 4 (i) requires significant power to stretch/compress the relatively stiff inductor; (ii) causes work hardening of the inductor which results in non-uniform inductance along the length of the coil; and (iii) requires a low-impedance, high-current collar which is subject to wear and potential breakdown over time.
The prior art matching mechanism shown in FIG. 4 is provided by the capacitor C.sub.C which provides the capacitive coupling of the RF signal input from connector 50 to the inductor L. As shown more clearly in FIG. 5, the capacitor C.sub.C comprises a C-shaped element 52 having adjustable extensions 54 attached thereto by screws 56. The capacitor C.sub.C functions as a transformer to match the impedance of the RF source (typically 50 .OMEGA.) with the impedance of the circuit R.sub.L (typically 1 M.OMEGA.). The adjustable extensions 54 may be extended or retracted to adjust the capacitance of capacitor C.sub.C. However, this matching mechanism provided in FIGS. 4 and 5 requires that the RF coupling to the inductor L be made at the high-voltage end of the inductor, thereby increasing the risk of arcing between the electrically grounded capacitor C.sub.C and the high-voltage inductor stem 44.
Accordingly, it is an object of the present invention to provide a resonator coil assembly having improved mechanisms for tuning and matching that overcome the deficiencies in the prior art. It is a further object to provide such a coil assembly for use in an ion implanter. It is yet a further object to provide methods and devices for tuning and matching such a coil assembly.